Skull pot for melting or refining inorganic substances

ABSTRACT

According to the invention, the skull pot is provided with the following characteristics: a pot wall ( 1 ), a bottom ( 3 ) and an induction coil ( 9 ) which surrounds the pot wall ( 1 ) and by means of which high-frequency energy can be coupled into the contents of the pot. The pot wall ( 1 ) is made of a ring of metal pipes ( 1.1 ) which can be connected to a cooling medium. Slits are embodied between adjacent metal pipes ( 1.1 ). The metal pipes ( 1.1 ) are bent at a right angle at the upper ends thereof in such a way that said pipes extend towards the outside, when the pot wall ( 1 ) is viewed from above, and form a collar ( 2 ). The collar ( 2 ) is surrounded by an additional wall (upper wall  4 ). The upper edge of said wall is situated on a higher level than the collar ( 2 ) in such a way that the melt covers the collar ( 2 ) during operation.

The invention relates to a so-called skull crucible for melting orrefining glasses or glass ceramics.

Troughs made of refractory material are strongly attacked by glasseswith high melting points above 1650° C., so that the dwell times becomeuneconomical and the produced glasses are full of stones, knots andstreaks from the trough material.

At temperatures above 1650° C., the use of an additional electricheating is strongly impaired because the corrosion of the electrodessuch as Mo-electrodes increases strongly and the glasses are stronglycolored by the impurities.

Aggressive glasses as are required for a number of optical applicationsfor example strongly attack the ceramic refractory materials during themelting, and especially melt-down, even at lower temperatures. Thestrong attack on the trough allows neither an economic meltingconcerning the service life of the troughs, nor a precise adherence tothe composition and, in connection with this, the required properties.That is why many of these glasses are molten in platinum troughs. Anumber of the aggressive glasses cannot be molten even in platinumcrucibles, because they attack platinum and the dissolved platinum oxidecolors the glass or the platinum oxide is reduced to platinum metalduring the further processes and causes disturbances as platinumparticles.

In high-purity glasses as are used in fiber optics for example, evenvery few ppb of coloring oxides which are introduced by the meltingprocess can be disturbing.

The heating of glass by means of high frequency offers the possibilityto couple the energy directly into the glass. Impurities by electrodecorrosion can thus be prevented. The U.S. Pat. No. 4,780,121 describes ahigh-frequency heated ceramic refining crucible in whichsoda-lime-silica glass is refined at temperatures of between 1150° C.and 1450° C. The disadvantage of this method is that the refractorymaterial is still very strongly attacked by the glasses at temperaturesover 1700° C.

As a result of the direct energy induction into the glass, the glassescan also be heated to temperatures above 1650° C. When using ceramiccrucible or trough materials, the temperature on the inner wall of thecrucible should not exceed 1650° C. In order to maintain thistemperature it is necessary that the temperature gradient in thecrucible wall must become increasingly steeper with rising temperatures,i.e. the crucible wall must become increasingly thinner and the coolingof the outer wall must become increasingly more intense. The cooling ofthe outer wall by natural convection as described in the U.S. Pat. No.4,780,121 is limited within narrow margins since arc-overs occur betweenthe crucible wall and the coil by the heated air. Higher meltingtemperatures can be achieved when the ceramic crucible is cooled bywater-cooled copper pipes.

In a number of patent specifications (U.S. Pat. No. 3,461,215, DE 2 033074, EP 0 119 877 B1, DE 3 316 546 C1) skull crucibles are described inwhich the ceramic inner crucible is omitted entirely. Meltingtemperatures of up to 3000° C. are reached. In literature, continuouslyoperating skull crucibles for melting radioactive materials aredescribed. By using the skull crucible, it is possible to avoidobtaining radioactively contaminated trough material. No requirementsare placed on the molten glass concerning the bubble quality.

DE 33 16 547 C2 describes a cold crucible for the melting ofnon-metallic organic compounds. A superstructure is placed on the uppercrucible edge which consists of oxide ceramics. This superstructure isof cylindrical shape. It is used for reducing heat losses.

The disadvantage of all skull crucible systems as described inliterature and the patents is that the water-cooled components reachinto the gas space above the melt surface. A number of substantialproblems are linked to this:

1. The melt surface is cooled by heat dissipation and the water-cooledskull crucible walls. This leads to a significant temperature gradientfrom the center to the surface of the melt. This is disadvantageous forthe application as a refining unit, because the bubbles cannot rise atall or only rise inadequately through the cold surface layer or strongfoam formation occurs.

2. When using an additional burner heating, the sulfur-containing burnerexhaust gases condensate on the cooled skull fingers and lead tocorrosion of the copper as a result of the formation of sulfuric acid.This drastically reduces the service life of the skull crucible.

3. In the case of aggressive glasses, corrosion of the water-cooledcopper components in the upper furnace chamber can occur. As a result ofdirect flaking of the corroded cooling finger surface or by conveyancevia the gas phase, the metallic impurities reach the glass melt and leadto discolorations of the melt.

The object of the present invention is to provide a high-frequencyheated skull crucible without a ceramic inner crucible for heating glassmelts to temperatures of up to 3000° C., preferably up to 2600° C., andthe glass surface to temperatures of up to 2600° C., preferably up to2400° C., and in which the metallic cooling fingers are protectedagainst corrosion by condensed combustion gases or evaporation products.

This object is achieved by the features of claim 1.

The following is achieved in detail by the invention: The coolingfingers are completely covered by the glass melt on the side facing theglass melt. They are thus protected against the exhaust gases orevaporation products from the hot glass surface.

This is achieved in such a way that the metallic cooling fingersconverge in the upper crucible zone, but underneath the glass surface,from the perpendicular into the horizontal. This convergence can be madegradually or the cooling pipes are bent by 90°. The bending of thecooling pipes into the horizontal leads to a cooled collar a shortdistance underneath the melt surface. The temperature of the glass meltdecreases outwardly in the zone of the collar. The glass melt can becooled off in the edge zone of the collar to such an extent that a ringmade of ceramic refractory material can be placed on the edge of thecollar. The temperature in the edge zone can be set in the edge zone viathe collar diameter and the glass level, so that even at very high melttemperatures in the core zone the glass can be cooled off in the outerzone and can be held by the refractory edge.

Corrosion problems on the metallic cooling fingers are thus avoided. Theservice life of the metal pipes and thus of the crucible per se isincreased by several times.

Furthermore, the glass surface is screened from the cooling fingers bythe melt per se. The melt prevents that the upper furnace chamber iscooled in an undesirable way by the cooling fingers. Higher temperaturescan thus be achieved in the upper furnace chamber in a controlled way,so that higher temperatures are also obtained in the surface layer ofthe melt. This is particularly advantageous during the refining. It isthus possible to either omit the addition of refining agents or therefining process can be performed in a shorter period.

The mushroom-like crucible shape in accordance with the invention is notonly advantageous during the refining but already during the melt-downprocess. Because the surface assumes higher temperatures than inconventional crucibles, there is a more rapid melt-down of the glassbatch. The throughput is thus increased as compared with knowncrucibles. A further advantage of the invention is that no corrosionproducts of the cooling finger can reach the glass melt.

The invention allows meeting all requirements for technical as well asoptical glasses, which especially includes the demand for favorabletransparency, whereby the glasses must be free from any bubbles.

During the refining with a mushroom-type crucible according to theinvention, the glass is liberated from physically and chemically boundgases. The refining process is supported during conventional glassmelting by refining agents such as N₂SO₄, As₂O₃, Sb₂O₃, or NaCl. Saidrefining agents decompose or evaporate at refining temperature and formbubbles into which the residual gases from the melt can diffuse. Therefining bubbles must be sufficiently large in order to rise to thesurface in the glass melt and burst there within economically viabletimes. The rising speed of the bubbles depends both on the bubble sizeand the viscosity of the glass. In the case of a temperature increasefrom 1600° C. to 2400° C., the rising speed increases approximately by afactor of 100, i.e. a bubble with a diameter of 0.1 mm rises at 2400° C.as fast as a bubble of 1 mm at 1600° C.

By increasing the refining temperature, the physical and chemicalsolubility is decreased in most gases and thus the high-temperaturerefining is additionally supported.

The high-temperature refining offers the possibility either to radicallyreduce the refining time or to omit the addition of refining agents forproducing large refining bubbles. The precondition is, however, that therising gas can reach the glass surface and the bubbles disposed at thesurface will burst and no foam is formed.

A particularly decisive advantage is thus the extraordinarily hightemperature which can be achieved with the invention.

The heating of the mushroom-type crucible in accordance with theinvention is carried out substantially by irradiation withhigh-frequency energy in the crucible zone below the collar. The meltsurface is considerably hotter in the upper furnace chamber due tothermal insulation than in the simple known cylindrical skull crucibles.

In the mushroom-type crucible in accordance with the invention, the meltsurface can be additionally heated by a gas burner or radiation heating.The exhaust gases from the burner cannot condensate on cold componentsin this arrangement. Instead, they are led off from the crucible zonevia an exhaust gas opening. The same applies to evaporation productsfrom the hot glass surface. As a result, there are no longer anycorrosion problems on the metallic cooling fingers and the mushroom-typecrucibles have a virtually unlimited service life.

The increase of the temperature in the melt surface by improvedinsulation of the upper furnace chamber or by additional heating withgas burners or radiation heating also produces an improved coupling ofthe high frequency into this zone, because the hotter glass surfacelayers have a higher conductivity than cold ones. A self-amplifyingeffect is thus obtained.

Improved results could also be achieved for the refining due to the hotmelt surface, because a hot glass surface is a precondition for aneffective bubble emission from the melt. Although the glass surfaceshows a descending temperature gradient towards the edges, the bubbleswhich are produced in the vertical part of the crucible and riseperpendicularly meet a hot glass surface. A rapid rise of bubbles andthus rapid bursting of the bubbles is ensured.

The invention is now explained in closer detail by reference to theenclosed drawings, wherein:

FIG. 1 shows a schematic view in an upright projection of the basicprinciple of a mushroom-type crucible.

FIG. 2 shows a top view of the metal pipes from which the collar isformed.

FIG. 3 shows a top view of a collar which is formed from plates.

FIG. 4 shows in a schematic representation an installation for meltingand refining glass in which the mushroom-type crucible is used formelting glass.

FIG. 5 shows in a schematic representation another installation formelting and refining glass in which the mushroom-type crucible is usedfor refining glass.

FIG. 6 shows a schematic view in an upright projection of the basicprinciple of a mushroom-type crucible with an outlet in the upper zone.

FIG. 7 shows in a schematic representation a further embodiment of aninstallation for melting and refining glass in which both the melting aswell as the refining are performed in a mushroom-type crucible each.

FIG. 8 shows in a schematic representation a further installation formelting and refining glass in which both the melting as well as therefining are performed in a mushroom-type crucible each.

The crucible shown in FIG. 1 is substantially shaped like a mushroom, asis shown. It comprises a cylindrical wall 1. It is formed by a ring ofvertical metal pipes. The vertical metal pipes are bent at their upperends by 90 degrees and form in their entirety a collar 2.

The bottom 3 of the crucible is made of refractory material which can belaid in bricks. The bottom can also be made of cooled metallic pipes orrings. This is particularly advantageous at very high meltingtemperatures. One can also see an outlet 3.1 for discharging thefinished glass melt.

An upper wall 4 is placed on the outer edge of collar 2. It is designedas a cylindrical ring of a ceramic refractory material. A cover 5 alsoconsists of refractory material. The upper furnace chamber 6 is enclosedby the upper wall 4, the cover 5 and the liquid level 7 of the melt.

The nozzle 8 of a burner projects into the upper furnace chamber 6.

An induction coil 9 is provided. It is used to couple high-frequencyenergy into the glass melt of the crucible.

In the lower zone of the crucible wall 1 there is an electricshort-circuit ring 10. It encloses the bottom 3. It concerns awater-cooled ring with which the bottom 3 of the mushroom-type crucibleis short-circuited. The short-circuit is necessary in order to preventarc formation at high melting temperatures. In very large cruciblesthere is an additional electric short-circuit ring 10 a above at thecollar edge.

Although the horizontal pipe sections 2.1 which form the collar 2 arebent rectangularly as compared with the perpendicular pipes 1.1, thisneed not necessarily be so. It is also possible that the pipe sections2.1 could extend under another angle, e.g. in such a way that they riseslightly from the inside to the outside.

FIG. 2 shows that the pipes 1.1 of the crucible wall 1 are arranged in aring-like manner and approximately form a cylinder.

FIG. 2 further shows the configuration of the pipes 2.1 of the collar 2.

FIG. 3 shows a top view of another arrangement of a collar 2. The collarconsists in this case of a plurality of hollow plates 2.2. They areconnected to the metal pipes 1.1 of the crucible wall 1. They can beflowed through by coolant alternatingly radially from the outside to theinside and from the inside to the outside.

Instead of the hollow plates 2.2 which are flowed through, it is alsopossible to provide the following construction: Plates are provided asshown in FIG. 3, but coolant does not flow directly through the plates.Instead, they enclose metal pipes which are flowed through.

The installation according to FIG. 4 shows a filling funnel 11 withwhich a melting crucible is supplied with a glass batch or refuse glass.The melting crucible again comprises the most essential components ofthe mushroom-type crucible according to FIG. 1, i.e. a crucible wall 1,a collar 2, a bottom 3, an upper wall 4, a cover 5 as well as aninduction coil 9.

After melting in the mushroom-type crucible, the melt reaches a refiningchamber 13 through a conduit 12 and finally a shaping station (notshown) via a conditioning reservoir 14 with a stirrer 14.1.

In the embodiment according to FIG. 5 the glass is molten in aconventional manner in a melting end which is made of bricks from arefractory material. Temperatures of up to 1700° C. are reached.

The melt arrives from below into a mushroom-type crucible 13 via aconnecting conduit 12 where the refining takes place. The mushroom-typecrucible is again enclosed by an induction coil 9. The arch above themelt level is further associated with a burner. In the mushroom-typecrucible, the arch is additionally heated with the burner or a pluralityof burners at melt temperatures of up to 1900° C. (core temperature ofthe melt) in order to ensure sufficient surface temperatures of morethan 1700° C. for the refining. At very high melt temperatures of over2000° C. it is necessary to actively cool the arch in order to avoidoverheating. Cooling is performed by injecting air or other gases intothe upper furnace chamber 6 or by cooling the arch with a liquid medium.The arch, like the crucible, is made from coolable metallic componentswhich, however, are lined with refractory materials in order to preventcorrosion by exhaust gases.

The glass melt leaves the refining crucible 13 by emerging to the sidein the zone of the collar. It reaches a cooling conduit 12.1 where it iscooled to temperatures of below 1700° C. A conditioning reservoir 14with a stirrer 14.1 is connected to the cooling conduit 12.1.

FIG. 6 shows a cross-section through a mushroom-type crucible with anoutlet at the side at the top.

In the embodiment according to FIG. 7, one recognizes a combination oftwo mushroom-type skull crucibles according to the invention. Bothoperate with high-frequency energy (see coils 9). The skull crucible Ais used as a melting unit and skull crucible B is used for refining.

The crucible A is supplied with the glass batch or glass melt fromabove. The molten glass is discharged at the crucible bottom. The glassmelt is supplied to skull crucible B from below via conduit 12. Theconduit is thus connected to the bottom of skull crucible A on the onehand and to the bottom of skull crucible B on the other hand. This leadsto the following advantage. It is achieved in this way that the surfacelayer of the glass melt in crucible B is relatively hot and the gasbubbles will thus rise up.

The dimensioning of a crucible model with approx. 8 L of hot melt volumeis mentioned as a concrete embodiment. The crucible has a diameter of 20cm in the lower zone. It is short-circuited at the bottom by awater-cooled ring. The melt level is 25 cm. The cooling fingers areoutwardly bent off by 90 degrees at a height of 20 cm. The collar has anoutside diameter of 50 cm. A ring of ceramic silicon dioxide orzirconium dioxide or zirconium silicate sits on the collar edge. Theglass sealing is made via the contact of the ceramic ring with thewater-cooled collar plate. The cover plate also consists of silicondioxide or zirconium dioxide or zirconium silicate. The heating of theupper furnace chamber is made by means of an oxygen burner.

The above crucible could be used both as a continuously operatingrefining crucible as well as a discontinuous melting crucible overseveral months without any corrosion problems occurring.

It is understood that larger volumes can be achieved by respectiveupscaling. In a crucible with a melt volume of 200 L, a second electricshort circuit on the outer collar edge has proven to be necessary.

In the embodiment according to FIG. 8, a refining crucible B is provideddownstream of melting crucible A. The melt reaches crucible B fromcrucible A in free fall. Both cases again concern the mushroom-typecrucible in accordance with the invention. The advantage of thisarrangement is that the connecting paths between the high-frequencycomponents are relatively short. This plays an important role whenaggressive glasses with high requirements placed on transmission are tobe produced. Resistance-heated platinum components are used asconnecting elements in this case.

The dimensioning of a crucible model with approx. 8 L of hot melt volumeis mentioned as a concrete embodiment. The crucible has a diameter of 20cm in the lower zone. It is short-circuited at the bottom by awater-cooled ring. The melt level is 25 cm. The skull cooling fingersare outwardly bent off by 90 degrees at a height of 20 cm. The collarhas an outside diameter of 50 cm. A ring of ceramic silicon dioxide sitson the collar edge. The glass sealing is made via the contact of theceramic ring with the water-cooled collar plate. The cover plate alsoconsists of silicon dioxide. The heating of the upper furnace chamber ismade by means of an oxygen burner which projects laterally into theupper furnace.

The coil has a distance of 2 cm from the skull crucible and 4 cm fromthe collar. The heating of the glass is performed by means ofhigh-frequency energy. The high-frequency is at 1 MHz. Thehigh-frequency output is between 100 and 300 kW, depending on themelting temperature.

The above crucible could be used both as a continuously operatingrefining crucible as well as a discontinuous melting crucible overseveral months without any corrosion problems occurring.

Larger volumes of the mushroom-type skull require a respective upscaleof the high-frequency output and an adjustment of the high frequency. Afrequency of 100 kHz and high-frequency outputs of 1000 to 2000 kW(depending on the desired temperature) are required for a mushroom-typecrucible with 400 liters of melt volume. A limitation of the melt volumeis essentially seen only by the maximum accessible high-frequencyoutput.

What is claimed is:
 1. A skull crucible for the melting or refining ofglasses; 1.1 with a crucible wall (1); 1.2 with a crucible bottom (3);1.3 with an induction coil (9) which encloses the crucible wall (1) andby which high frequency energy can be coupled into the content of thecrucible; 1.4 the crucible wall is formed by a ring of metal pipes (1.1)which can be connected to a cooling medium, with slots between mutuallyadjacent metal pipes (1.1); 1.5 the metal pipes (1.1) are bent off attheir upper ends in such a way that, in a top view of the crucible wall(1), they extend outwardly and form a collar (2); 1.6 the collar (2) isenclosed by a further upper wall (4) whose upper edge projects beyondthe plane of the collar (2), so that the melt covers the collar (2)during operation.
 2. A skull crucible as claimed in claim 1,characterized in that the upper furnace chamber (6) is covered above themelt.
 3. A skull crucible as claimed in claim 1, characterized in thatthe upper furnace chamber (6) is associated with one or several burners(8).
 4. A skull crucible as claimed in claim 1, characterized in thatthe metal pipes (1.1) expand after the bend-off in the zone of thecollar (2) into hollow plates (2.2) which carry cooling medium directlyor indirectly.
 5. A skull crucible as claimed in claim 1, characterizedin that the metal pipes (1.1) are enclosed by hollow plates in the zoneof the collar (2).
 6. A skull crucible as claimed in claim 4,characterized by the following features: 6.1 the plates (2.1) aretrapezoid (as seen in a top view); 6.2 the plates (2.1) are arranged anddisposed in such a way that a radially extending slot remains betweentwo mutually adjacent plates.
 7. A skull crucible as claimed in claim 5,characterized in that the slots are provided with a constant width.
 8. Askull crucible as claimed in claim 1, characterized in that the upperwall (4) is made of ceramic material and that no water-cooled metalliccomponents are disposed in the upper furnace chamber above the melt. 9.A skull crucible as claimed in claim 1, characterized in that thecrucible can be filled from above and has an outlet in the bottom.
 10. Askull crucible as claimed in claim 1, characterized in that the skullcrucible has an inlet in the bottom and an outlet in the upper part. 11.A skull crucible as claimed in claim 1, characterized in that the outletis a resistance-heated platinum pipe.
 12. A skull crucible as claim inclaim 2, characterized in that the upper furnace chamber (6) isassociated with one or several burners (8).
 13. A skull crucible asclaimed in claim 2, characterized in that the metal pipes (1.1) expandafter the bend-off in the zone of the collar (2) into hollow plates(2.2) which carry cooling medium directly or indirectly.
 14. A skullcrucible as claimed in claim 3, characterized in that the metal pipes(1.1) expand after the bend-off in the zone of the collar (2) intohollow plates (2.2) which carry cooling medium directly or indirectly.15. A skull crucible according to claim 2, characterized in that themetal pipes (1.1) are enclosed by hollow plates in the zone of thecollar (2).
 16. A skull crucible according to claim 3, characterized inthat the metal pipes (1.1) are enclosed by hollow plates in the zone ofthe collar (2).
 17. A skull crucible as claimed in claim 5,characterized by the following features: the plates (2.1) are trapezoid(as seen in atop view); the plates (2.1) are arranged and disposed insuch a way that a radially extending slot remains between two mutuallyadjacent plates.
 18. A skull crucible as claimed in claim 2,characterized in that the upper wall (4) is made of ceramic material andthat no water-cooled metallic components are disposed in the upperfurnace chamber above the melt.
 19. A skull crucible as claimed in claim3, characterized in that the upper wall (4) is made of ceramic materialand that no water-cooled metallic components are disposed in the upperfurnace chamber above the melt.
 20. A skull crucible as claimed in claim4, characterized in that the upper wall (4) is made of ceramic materialand that no water-cooled metallic components are disposed in the upperfurnace chamber above the melt.